1. Field of the Invention
This invention relates to a semiconductor roll exhibiting a stable electric resistivity in the semiconductive range of 10.sup.3 to 10.sup.10 ohm-cm.
2. Prior Art
A number of electroconductive rubbers are available as mixtures of electrically insulating rubber material and electroconductive agents. For example, carbon black and other conductive agents are blended in rubbers to provide conductive rubbers with an electric resistivity of from 10.sup.-1 to 10.sup.2 .OMEGA.-cm, which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its heat resistance, low temperature resistance and weatherability. It can also be used as a conductive silicone rubber by adding conductive agents such as other rubber materials.
The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and short fibers surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing these conductive agents, the resistivity of silicone rubber can be reduced to the order of 10.sup.10 to 10.sup.-3 .OMEGA.-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber.
When silicone rubber is blended with conductive carbon black such as Ketjen Black and acetylene black, however, the resulting compositions show great resistivity variations in the semiconductive region of 10.sup.3 to 10.sup.10 .OMEGA.-cm. It is difficult to provide stable electrical resistivity probably because the dispersion of carbon considerably varies with molding conditions.
In the manufacture of business machine parts including those used in dry copying machines, for example, rubber rolls such as rolls for charging, transfer, developing, paper feed, fixing, pressing, electricity removal, cleaning, and oil application, the industry is now searching for materials having semiconductive characteristics. There is a need for a semiconductive roll which has stable resistivity with minimal variation in the semiconductive region.